Project Summary Opiate abuse and overdose have risen to epidemic proportions in the USA in recent years. Only limited medication options are currently available, which is in large part due to the surprising lack of knowledge about the pathophysiologic mechanisms that underlie opiate addiction. Recent animal studies have provided robust evidence that repeated drug exposure induces changes in gene expression through alterations in epigenetic regulation that are linked to addiction-related behavioral abnormalities. However, information about the epigenetic landscape in the brains of human addicts remains limited and is critical for the development of clinically effective treatments. Recent studies have achieved a comprehensive mapping of brain-specific epigenetic marks in the human genome using homogenate postmortem tissue. However, cellular heterogeneity of the brain precludes a reliable annotation of cell-type-specific epigenetic modifications from these data. Aim1 of this project will leverage our newly developed molecular strategies to illuminate the neural subtype-specific epigenome of heroin addiction with unprecedented detail?in four different populations of brain cells?which will highly enhance the likelihood of identifying cell-type-specific signatures of addiction-associated epigenetic variations. Our recent epigenetic studies in glutamatergic (Glu) projection neurons and inhibitory GABA interneurons from the human orbital frontal cortex (OFC) suggest that many activity-dependent genes (ADGs) are ?poised? to be activated in a neuron-subtype-specific manner. ADGs are activated by experience-driven synaptic activity (including exposure to addictive drugs) and regulate diverse aspects of the nervous system, (including synaptic plasticity). Notably, our recent RNA-seq data in homogenate OFC samples showed that among the most significant changes is the downregulation of a subset of the ADGs. These results are in line with the growing body of literature that implicates diminished output from the ventral aspects of the prefrontal cortex in drug addiction. We hypothesize that the steady-state expression and inducibility of the ADGs (the latter is determined by their epigenetic milieu) are altered in heroin addicts in a neuron-subtype-specific manner, and we will test this hypothesis in Aim 2. Considering the importance of ADGs in synaptic plasticity that accompanies the development and maintenance of drug addiction, in Aim 2 we will utilize a translational animal model to explore the cell-specific functional contribution of ADGs to heroin self-administration behavior. Overall, this innovative line of research will develop unique datasets that will be available to the research community and will provide an urgently needed resource for genome-wide neural subtype-specific chromatin and transcriptome maps in heroin abuse and normal subjects. Moreover, the mechanistic studies in animal models can promote the development of novel therapeutic interventions.